The invention relates to electroerosion printing and to recording materials characterized by an improved hydrophilic, conductive or resistive lubricant topcoat, especially for use in the production of direct offset masters or direct negative applications.
Electroerosion printing is a well-known technique for producing markings, such as, letters, numbers, symbols, patterns, such as, circuit patterns, or other legible or coded indicia on recording material in response to an electric signal which removes or erodes material from the surface of the recording material as the result of spark initiation.
The surface which is eroded or removed to provide such indicia on the recording material is usually a thin film of conductive material which is vaporized in response to localized heating associated with sparking (arcing) initiated by applying an electric current to an electrode in contact with the surface of a recording material comprising the thin conductive film on a flexible nonconductive backing or support. In the present state of the technology the thin conductive film is usually a thin film of vaporizable metal, such as, aluminum.
Electroerosion printing is effected by the movement of a stylus or a plurality of styli relative to the surface of specially prepared recording media. Electrical writing signals are fed to the stylus to provide controlled electrical pulses which generate sparks at the surface of the recording material to selectively heat and remove by evaporation a layer of the recording material; the locations from which material is removed correspond to the indicia or images which are to be recorded.
In the course of this process, the stylus is moved relatively to a surface of the recording material and in contact with the removable layer, e.g., a thin film of vaporizable material, usually a metal, such as aluminum.
Due to the fragility of the thin conductive layer and stylus pressure, considerable scratching (undesired removal of the removable layer) is observed to take place during electroerosion printing.
It has been recognized for some time, therefore, that the use of a lubricant and/or protective overcoat on the surface of such electroerosion recording materials would be helpful to reduce scratching by the stylus. After some investigation, lubricants comprising long chain fatty acids were adopted. Even with the use of such lubricants, however, some stylus scratching of the thin aluminum film of electroerosion recording materials continues to be observed. Therefore, efforts continued to be directed to finding a superior lubricant--protective layer composition for the surface of electroerosion recording materials.
U.S. Pat. No. 4,622,262 describes the use of graphite dispersed in a water insoluble polymeric binder, for example, ethyl cellulose. Although the corrosion resistance of the film is very good, the material cannot readily be used as direct offset plate because the surface of the material is hydrophobic and the top coat can not be easily removed from the surface.
U.S. Pat. No. 4,617,579 describes the use of a hydrophilic-water soluble binder (hydroxy ethyl cellulose or the like) in the top coat. In this case although the material can be used both as negative and plate, because of the conductivity, hydrophilicity and water solubility of the top coat, it undergoes rapid corrosion of the aluminum film from fingerprinting and in humid environments. Therefore, after handling or short storage times it could not reliably be used as direct negative/direct plate (DNP).
U.S. Pat. No. 4,567,490 describes the use of fluorinated graphite and Teflon with or without addition of graphite. Although the corrosion resistance of DNP materials coated with these fluorinated compounds increases due to their low conductivity, the print quality is poor.